For nearly a century X-rays for medical and technological use have been generated using bremsstrahlung and characteristic line emission. The intensity of this radiation is relatively weak for many commercial and medical applications. This is especially true for moving mechanical systems (e.g. gear trains) and biological tissue (e.g. arteries of the heart). In the past twenty years a brighter more collimated X-ray source from synchrotron emission has been used to generate both hard X-rays and soft X-rays for scientific and technological research. For example, very recent work using X-ray synchrotron emission from electron storage rigs offers the prospect of a new method of non-invasive coronary angiography (medical imaging of the arteries of the heart, see Hughes et al., "The application of synchrotron radiation to non-invasive angiography," Nuc. Instrum. Meth., vol. 208, p. 665, 1983). The high intensity and collimation of the synchrotron radiation permit the X-rays to be Bragg-diffracted so that only a narrow band of energies remain. The selected energy of the X-rays are subject to fine adjustment by small changes in the Bragg angle allowing digital subtraction of the X-ray images acquired at energies slightly above and below that of the iodine k-shell-photoabsorption edge at 33.16 keV, the iodine having been injected into the bloodstream intraveniously. This digital subtraction, called dichromography, substantially eliminates all image contrast due to other body structures and thereby achieves maximum contrast between the iodinated arteries and the surrounding tissue. Furthermore, when using the scanning method, the intensity of the synchronotron X-ray beams is such that the pairs of one-dimensional images, above and below the k-edge, can be recorded in a very short time. In this way, the prospect of visualizing the coronary arteries without motion artifacts is achieved. A conventional X-ray tube is generally not bright enough or collimated enough to achieve this kind of imaging in such a short time.
Unfortunately, the large storage rings with periodic magnetic fields for the generation of synchrotron radiation are presently extremely expensive. Estimated costs for such facilities are between 10 and 25 million dollars. A cheaper source is clearly needed.
Another source of X-rays is transition radiation from thin foils using electrons from high-current linear accelerators. Transition radiation occurs when charged particles encounter a sudden change in dielectric constant at the interface between dissimilar media (e.g. between a vacuum and a solid). Conservation of energy and momentum requires that a cone of X-rays be emitted.
In the prior art transition radiation has only been applied to high-energy-particle detection. Previously only low-density foils were used (densities&lt;2.25 gm/cm.sub.3), and, in order to raise the output photon frequency, the electron-beam energy was raised. For example, electron energies of 2 GeV or more were used with low-density foils such as mylar, lithium and beryllium. (see M. L. Cherry et al. "Transition radiation from relativistic electrons in periodic radiators," Phys. Rev. D vol. 10, pp. 3594-3607, December 1974.)
Transition radiation has also been considered as a source of soft X-rays (photon energy&lt;2 keV) using low density (.rho.&lt;3 gm/cm.sup.3 ) foils for lithography (see M. A. Piestrup et al. "Measurement of transition radiation from medium energy electrons", Phys. Rev. A, vol. 32. pp. 917-927, August 1985).